Should You Ever Eat Below Your BMR?

The Short Answer

In general, it's best to avoid eating significantly below your BMR for extended periods. However, short-term dips below BMR (1-2 weeks) or slight deficits (50-100 calories below BMR) are usually fine for fat loss.

The key is 'how far below' and 'for how long.' Eating 100 calories below your BMR for a month is very different from eating 500 calories below your BMR for 6 months.

Why Eating Way Below BMR Is Risky

Your BMR represents the minimum energy your body needs to keep you alive—heart beating, lungs breathing, brain thinking, cells repairing. When you eat well below this for a long time:

Your metabolism slows down significantly (metabolic adaptation)

You lose muscle mass, which further lowers your BMR

Energy levels plummet, making you tired and unproductive

Hormones get disrupted (thyroid, sex hormones, stress hormones)

For women: periods may become irregular or stop completely

Immune function weakens, making you sick more often

Mood disorders like depression and anxiety can develop or worsen

Your body essentially goes into 'survival mode,' prioritizing basic functions and shutting down non-essential processes.

When Eating Below BMR Might Be Okay

There are a few scenarios where eating slightly below BMR is acceptable:

Scenario 1: You're very sedentary. If your TDEE is only 1.1-1.2× your BMR (because you truly don't move much), a small deficit might put you slightly below BMR. This is okay short-term if you're still eating a reasonable amount (1,200+ calories for women, 1,500+ for men).

Scenario 2: You're under medical supervision. Doctors sometimes prescribe very low-calorie diets (VLCDs) for people with obesity-related health risks. This should only be done with professional monitoring.

Scenario 3: It's temporary. If your deficit occasionally dips 50-100 calories below BMR for a week or two, it's not catastrophic. The problems arise with prolonged, severe deficits.

Scenario 4: You're tracking carefully and feeling good. If you're eating slightly below BMR but have high energy, good sleep, stable mood, and consistent progress, you're probably fine. Listen to your body.

What To Do Instead

Rather than eating far below your BMR, try these strategies:

Option 1: Eat at or slightly above BMR, and increase physical activity to create your deficit. For example, eat 1,500 calories (your BMR) and burn an extra 300 through walking or exercise.

Option 2: Use a smaller calorie deficit from your TDEE, not BMR. If your TDEE is 1,800, eat 1,500 (300-calorie deficit). This keeps you above your BMR of 1,400.

Option 3: Take diet breaks. Every 2-3 months, spend 1-2 weeks eating at maintenance (TDEE) to give your metabolism a break and restore hormones.

Option 4: Focus on building muscle through strength training. More muscle = higher BMR = easier to lose fat while eating more food.

These approaches are more sustainable and protect your long-term metabolic health.